The present invention relates to a high strength steel sheet that is useful as a member for automobiles, construction, electric devices and the like and to a process for its manufacture, and more particularly it relates to a high strength hot-dip galvanized steel sheet with excellent draw ability properties for press forming and excellent plating adhesion, to a hot-dip galvannealed plated steel sheet, and to a process for its manufacture.
A current area of research on members such as cross members and side members of automobiles and the like is directed toward achieving lighter weight for the purpose of realizing lower fuel consumption and, in the area of materials, progress is being made in achieving higher strength in a manner that guarantees strength while producing thinner products. However, since the press formability of most materials is inferior with increasing strength, achieving such lighter weight members requires the development of steel sheets that exhibit both satisfactory press formability and high strength properties. The index values for formability include the elongation as well as the n value and r value obtained in tensile testing, and in the current situation, where one of the targets is to simplify the pressing steps through greater integration, it is important for the n value to be large, thus corresponding to uniform elongation.
For this purpose, there have been developed hot rolled steel sheets and cold rolled steel sheets that take advantage of the transformation-induced plasticity of the retained austenite in steel. These are steel sheets consisting of about 0.07-0.4% C, about 0.3-2.0% Si and about 0.2-2.5% Mn as the basic alloy elements, without any expensive alloy elements, and containing retained austenite in the microstructure by heat treatment characterized by accomplishing bainite transformation at an inner and outer temperature of 300-450xc2x0 C. after annealing in the two-phase region; such steel sheets are disclosed, for example, in Japanese Unexamined Patent Publication No. 1-230715 and No. 2-217425. Such steel sheets are disclosed not only as cold rolled steel sheets manufactured by continuous annealing, but also as hot rolled steel sheets obtained by controlling the cooling and coiling temperature with a runout table, as in Japanese Unexamined Patent Publication No. 1-79345, for example.
The plating of automobile members is advancing for the purpose of improving corrosion resistance and outer appearance to reflect higher quality in automobiles, and at the current time galvanized or galvannealed steel sheets are used for most members other than special internally-mounted members. From the standpoint of corrosion resistance, therefore, it is effective to coat such steel sheets with Zn or Fe-Zn, however, since high strength steel with a high Si content also has an oxidation film on the steel sheet surface, it presents a problem in that minute unplated regions result upon hot-dip galvanizing, and in that the plating adhesion of worked regions after alloying is inferior; at the current time, however, it has not been possible to realize galvannealed steel sheets with excellent plating adhesion at worked regions, excellent corrosion resistance and high strength and high ductility.
For example, since the steel sheets disclosed in Japanese Unexamined Patent Publication No. 1-230715 or No. 2-217425 contain 0.3-2.0% added Si and take advantage of its unique bainite transformation to guarantee retained austenite, unless rather strict control is kept on the cooling after annealing in the two-phase temperature range, and the holding at an internal temperature of 300-450xc2x0 C., it is impossible to obtain the intended microstructure and the resulting strength and elongation are outside of the target ranges. While such a thermal history can be realized industrially with continuous annealing equipment and during the cooling step with the runout table after hot rolling, the austenite transformation is completed rapidly at 450-600xc2x0 C. and therefore control is required for a particularly short holding time at 450-600xc2x0 C. Even at 350-450xc2x0 C., the microstructure varies considerably depending on the holding time, and any shift from the desired conditions results in only an low level of strength and elongation. For hot-dip galvanizing the holding time at 450-600xc2x0 C. is usually long, and therefore this technique cannot be applied. Furthermore, there is a problem in that the inclusion of Si as the alloy element results in poorer plating, and this impedes passage through the hot-dip galvanizing equipment to make a plated steel sheet.
In order to solve these problems, there have been disclosed steel sheets with improved plating properties through restriction of the Si concentration, for example, in Japanese Unexamined Patent Publication No. 5-247586 and Japanese Unexamined Patent Publication No. 6-145788. According to such processes, Al is added instead of Si to produce retained austenite. However, since Al like Si is also more easily oxidized than Fe, the Al and Si tend to concentrate on the steel sheet surface as an oxidized film, making it impossible to achieve adequate plating adhesion. Another process disclosed in Japanese Unexamined Patent Publication No. 5-70886 adds Ni to improve the plating wettability. For this process, however, it is not disclosed what relationship between the Si or Al and Ni is necessary to inhibit the plating wettability.
Furthermore, Japanese Unexamined Patent Publication No. 4-333552 and No. 4-346644 disclose processes whereby rapid low temperature heating is conducted after Ni pre-plating for alloying treatment after hot-dip Zn plating, as processes for hot-dip galvannealing of high Si high strength steel sheets. However, since these processes require Ni pre-plating, the problem of requiring new equipment arises. These processes also do not allow retention of retained austenite in the final microstructure, nor is any such mention made for this process.
The present invention solves the aforementioned problems and represents the discovery of the features of the composition and microstructure of a high strength steel sheet with improved surface corrosion resistance and excellent plating adhesion allowing its manufacture with hot-dip galvanizing equipment, as well as satisfactory press formability.
It is an object of the present invention to provide a high strength hot-dip galvannealed steel sheet with satisfactory press formability and plating adhesion and a process for efficient manufacture of the steel sheet.
The present inventors have carried out diligent research on plating properties and steel sheet components in order to provide a high strength hot-dip galvannealed steel sheet and a process for its manufacture, and have completed the present invention by concentrating on the steel sheet surface layer, to arrive at the gist of the invention as described below.
(1) A high strength hot-dip galvannealed steel sheet with excellent plating adhesion and press formability, the steel sheet containing, in terms of weight percent,
C: 0.05-0.2%,
Si: 0.2-2.0%,
Mn: 0.2-2.5%,
Al: 0.01-1.5%,
Ni: 0.2-5.0%,
P:  less than 0.03% and
S:  less than 0.02%
where the relationship between Si and Al is such that 0.4(%) xe2x89xa6Si+0.8 Al(%) xe2x89xa62.0% and the remainder consists of Fe and unavoidable impurities, characterized in that the volume percentage of the retained austenite in the steel sheet is 2-20%, and the steel sheet surface wherein the relationship between the Ni and Si, Al in 0.5 xcexcm of the steel sheet surface layer is such that Ni(%) xe2x89xa7xc2xc Si+⅓ Al(%) has a Zn plating layer comprising Al: xe2x89xa61% with the remainder Zn and unavoidable impurities.
(2) A high strength hot-dip galvanized steel sheet with excellent plating adhesion and press formability, characterized by also containing, in addition to the steel sheet components mentioned in (1) above, in terms of weight percent, Cu at less than 2.0%, wherein the volume percentage of the retained austenite in the steel sheet is 2-20%, and the relationship between the Ni, Cu and Si, Al in 0.5 xcexcm of the steel sheet surface layer is such that Ni+Cu(%) xe2x89xa7xc2xc Si+⅓ Al(%).
(3) A high strength hot-dip galvanized steel sheet with excellent plating adhesion and press formability, characterized by also containing, in addition to the steel sheet components mentioned in (2) above in terms of weight percent, B at 0.0002-0.01%, wherein the relationship of Cu and B is such that Bxc3x97Cu(%) xe2x89xa70.00005(%).
(4) A high strength hot-dip galvanized steel sheet with excellent plating adhesion and press formability, characterized by also containing, in addition to the steel sheet components mentioned in any of (1) to (3) above in terms of weight percent, at least one from among Co at  less than 0.3% and Sn at  less than 0.3%, wherein the volume percentage of the retained austenite in the steel sheet is 2-20% and the relationship between the Ni, Cu, Co, Sn and Si, Al in 0.5 xcexcm of the steel sheet surface layer is such that Ni+Cu+Co+Sn(%) xe2x89xa7{fraction (1/4)} Si+⅓ Al(%).
(5) A high strength hot-dip galvanized steel sheet with excellent plating adhesion and press formability, characterized by also containing, in addition to the steel sheet components mentioned in any of (1) to (4) above in terms of weight percent, at least one from among Mo:  less than 0.5%, Cr:  less than 1%, V:  less than 0.3%, Ti:  less than 0.06%, Nb:  less than 0.06%, REM:  less than 0.05%, Ca:  less than 0.05%, Zr:  less than 0.05%, Mg:  less than 0.05%, Zn:  less than 0.02%, W:  less than 0.05%, As:  less than 0.02%, N:  less than 0.03% and O:  less than 0.05%.
(6) A high strength hot-dip galvanized steel sheet with excellent plating adhesion and press formability, characterized in that the steel sheet surface of any of (1) to (5) above has a Zn plating layer containing at least one from among Al: xe2x89xa61%, Mn:  less than 0.02%, Pb:  less than 0.01%, Fe:  less than 0.2%, Sb:  less than 0.01%, Ni:  less than 3.0%, Cu:  less than 1.5%, Sn:  less than 0.1%, Co:  less than 0.1%, Cd:  less than 0.01% and Cr:  less than 0.05%, with the remainder Zn and unavoidable impurities.
(7) A high strength hot-dip galvannealed steel sheet with excellent press formability, characterized in that a steel sheet containing in terms of weight percent,
C: 0.05-0.2%,
Si: 0.2-2.0%,
Mn: 0.2-2.5%,
Al: 0.01-1.5%,
Ni: 0.2-5.0%,
P:  less than 0.03% and
S:  less than 0.02%,
where the relationship between Si and Al is such that 0.4(%) xe2x89xa6Si+0.8 Al(%) xe2x89xa62.0%, the remainder consists of Fe and unavoidable impurities, the volume percentage of the retained austenite in the steel sheet is 2-20% and the relationship between the Ni and Si, Al in 0.5 xcexcm of the steel sheet surface layer is such that Ni(%) xe2x89xa7xc2xc Si+⅓ Al(%), has a Zn alloy plating layer comprising Fe: 8-15%, Al: xe2x89xa61% with the remainder Zn and unavoidable impurities.
(8) A high strength hot-dip galvannealed steel sheet with excellent press formability, characterized in that a steel sheet also containing, in addition to the steel sheet components mentioned in (6) above in terms of weight percent, Cu at less than 2.0% with the remainder Fe and unavoidable impurities, wherein the volume percentage of the retained austenite in the steel sheet is 2-20% and the relationship between the Ni, Cu and Si, Al in 0.5 xcexcm of the steel sheet surface layer is such that Ni+Cu(%) xe2x89xa7xc2xc Si+⅓ Al(%), has a Zn alloy plating layer comprising Fe at 8-15% and Al at xe2x89xa61% with the remainder Zn and unavoidable impurities.
(9) A high strength hot-dip galvannealed steel sheet with excellent press formability, characterized in that a steel sheet also containing, in addition to the steel sheet components mentioned in (7) above in terms of weight percent, B at 0.0002-0.01% where the relationship between Cu and B is such that Bxc3x97Cu(%) xe2x89xa70.00005(%) with the remainder Fe and unavoidable impurities, has a Zn alloy plating layer comprising Fe at 8-15% and Al at xe2x89xa61% with the remainder Zn and unavoidable impurities.
(10) A high strength hot-dip galvannealed steel sheet with excellent press formability, characterized in that a steel sheet also containing, in addition to the steel sheet components mentioned in any of (7) to (9) above in terms of weight percent, at least one from among Co at  less than 0.3% and Sn at xe2x89xa60.3% with the remainder Fe and unavoidable impurities, wherein the volume percentage of the retained austenite in the steel sheet is 2-20% and the relationship between the Ni, CU, Co, Sn and Si, Al in 0.5 xcexcm of the steel sheet surface layer is such that Ni+Cu+Co+Sn(%) xe2x89xa7xc2xc Si+⅓ Al(%), has a Zn alloy plating layer comprising Fe at 8-15% and Al at xe2x89xa61% with the remainder Zn and unavoidable impurities.
(11) A high strength hot-dip galvannealed steel sheet with excellent press formability, characterized by also containing, in addition to the steel sheet components mentioned in any of (7) to (10) above in terms of weight percent, at least one from among Mo:  less than 0.5%, Cr:  less than 1%, V:  less than 0.3%, Ti:  less than 0.06%, Nb:  less than 0.06%, REM:  less than 0.05%, Ca:  less than 0.05%, Zr:  less than 0.05%, Mg:  less than 0.05%, Zn:  less than 0.02%, W:  less than 0.05%, As:  less than 0.02%, N:  less than 0.03% and O:  less than 0.05%.
(12) A high strength hot-dip galvannealed steel sheet with excellent plating adhesion and press formability, characterized in that the steel sheet surface of any of (7) to (11) above has a Zn plating layer containing at least one from among Fe: 8-15%, Al: xe2x89xa61%, Mn:  less than 0.02%, Pb:  less than 0.01%, Sb:  less than 0.01%, Ni:  less than 3.0%, Cu:  less than 1.5%, Sn:  less than 0.1%, Co:  less than 0.1%, Cd:  less than 0.01% and Cr:  less than 0.05%, with the remainder Zn and unavoidable impurities.
(13) A process for manufacture of a high strength hot-dip galvanized steel sheet with excellent plating adhesion and press formability characterized by having 2-20% retained austenite and a Zn plating layer comprising Al at xe2x89xa61% with the remainder Zn and unavoidable impurities, whereby a steel sheet having the components of any one of (1) to (6) above is cast and solidified and then heated at 1150xc2x0 or higher for at least 45 minutes, after which it is subjected to hot rolling and coiling at 400-780xc2x0 C., and then after descaling treatment is subjected to cold rolling at a 35-85% draft, subsequently annealed from 10 seconds to 6 minutes in the two-phase temperature range of 650-900xc2x0 C., and finally cooled to 350-500xc2x0 C. at a cooling rate of 2-200xc2x0 C./s, subjected to hot-dip zinc plating and then cooled to below 250xc2x0 C. at a cooling rate of at least 5xc2x0 C./s.
(14) A process for manufacture of a high strength hot-dip galvannealed steel sheet with excellent press formability characterized by having 2-20% retained austenite and a Zn alloy plating layer comprising Fe at 8-15% and Al at xe2x89xa61% with the remainder Zn and unavoidable impurities, whereby a steel sheet having the components of any one of (7) to (12) above is cast and solidified and then heated at 1150xc2x0 or higher for at least 45 minutes, after which it is subjected to hot rolling and coiling at 400-780xc2x0 C., and then after descaling treatment is subjected to cold rolling at a reduction ratio of 35-85%, subsequently annealed from 10 seconds to 6 minutes in the two-phase temperature range of 650-900xc2x0 C., and finally cooled to 350-500xc2x0 C. at a cooling rate of 2-200xc2x0 C./s, and held in a temperature range of 450-600xc2x0 C. for 5 seconds to 1 minute prior to cooling to below 250xc2x0 C. at a cooling rate of at least 5xc2x0 C./s.
(15) A process for manufacture of a high strength hot-dip galvannealed steel sheet with excellent press formability according to (13) or (14) above, characterized in that the cold rolling is followed by annealing from 10 seconds to 6 minutes in the two-phase temperature range of 650-900xc2x0 C. and then by cooling to 350-500xc2x0 C. at a cooling rate of 2-200 xc2x0 C./s and held in that temperature range for no more than 5 minutes.